There are several times during the year when the Church invites her people to fast. The most significant of these is the period before Easter. Many begin this fast on Ash Wednesday but some, particularly among those in religious orders, begin this fast at Septuagesima. The reason for these different periods of fasting is explained in BOOK VI of the RATIONAL DIVINORUM OFFICIORUM in the chapter The Sunday of Septuagesima.
The Prophet Elijah Receiving Bread and Water from an Angel